Dimensionless parameter controlling the shape of the yield surface cap

default: $\xi = f_c/2p_0$

bulk

Flag to activate bulking

options: 0 $\rightarrow$ no bulking 1 $\rightarrow$ bulking activated

Description

This is a concrete model with different failure mechanisms in compression and tension. The material is assumed to
have a pressure dependent shear resistance. Inelastic deformation is a combination of shearing and dilatation.
Inelastic dilatation is interpreted as crushing that gradually reduces the shear resistance of the material.
Deviatoric inelastic strains eventuelly lead to the formation of macroscopic cracks. Node splitting is used
for the representation of such cracks.
Note that node splitting can not occur in MERGE or REFINE interfaces.

The total stress $\sigma$ is the sum of an elastic component $\sigma^e$ and a viscous component $\sigma^v$.

$\sigma = \sigma^e + \sigma^v$

where:

$\sigma^e = 2G \epsilon_{dev}^e - p \mathbf{I}$

$\epsilon_{dev}^e$ is the deviatoric elastic strain and $p$ is the pressure. The viscous stress component is defined as:

$\sigma^v = c \dot\epsilon$

where $\dot\epsilon$ is the total strain rate.
Note that the flow criteria (in both tension and compression) are evaluated using the elastic stresses.
In hydrostatic loading the elastic bulk modulus $K$ and the compaction pressure $p_c$ are
assumed to grow with the ineastic compaction strain $\epsilon_v^p$:

$\theta$ is the Lode angle, as define in the figure below. $D_t$ is the tensile damage, $D_c$ is the crushing damage and
$D_0$ is the initial defect level. Note that $p_s=0$ for a fully damaged material. Also note the shear rate
hardening, controlled by the parameters $r_s$ and $\dot\epsilon_{s0}$.
Initial defects are optional and can be defined with the command INITIAL_DAMAGE_RANDOM.